In reference specifically to optical fibers made from preforms, optical fibers made from heavy metal fluoride glass, such as ZrF.sub.4 -based glasses, possess a theoretical minimum attenuation of about 0.01 dB/km at around 2.5 .mu.m. This is over an order of magnitude lower than the 0.15 dB/km obtained for silica based optical fibers. While short lengths of the fibers made from heavy metal fluoride glass have demonstrated losses approaching theoretical values, fibers longer than 100 m exhibit losses of about 1 db/km. The main reason for this is due to scattering of light from defects such as crystals and bubbles and also particles of crucible material from crucibles used for melting the glass components.
Glass, particularly halide-containing glass, is made by melting high purity glass components in vitreous carbon crucible under an oxidizing gaseous atmosphere. In the case of heavy metal fluoride glass, the melting temperature is about 850.degree. C., the gaseous atmosphere is a SF.sub.6 N.sub.2 gas mixture, and duration of the melting processing in the gaseous atmosphere is about 2 hours. This melting schedule produces zero crystal growth on subsequent heat treatment of the glass and the resulting glass has an extremely low hydroxyl (OH) concentration of less than 3 ppb.
Unfortunately, the use of vitreous carbon crucibles in glass manufacture is deleterious when used to cast fiber preforms due to carbon contamination from the crucible walls above the melt surface. Although such preform melts are initially made in carbon crucibles, they are quenched and the glass is then remelted in platinum crucibles under an oxidizing atmosphere of O.sub.2 /N.sub.2 gas mixture and subsequently cast into preforms for fiber drawing. The oxidizing atmosphere removes a substantial portion of carbon as gaseous CO.sub.2, however, at least some of the carbon remains. It has been observed, however, that some crucibles leave a carbon film and carbon occlusions in sporadic regions on the glass surface. This occurrence is more prevalent in new crucibles which usually exhibit passivation after several melt schedules. However, some crucibles persistently give this problem of undesired carbon deposition.
During remelting of the glass, the undesired carbon deposition is introduced into the bulk of the glass producing carbon occlusions in the preforms or whatever is made from the glass. Subsequent remelting, drawing or any reshaping of the glass, leads to undesirable crystallization if impurities or imperfections are present in the glass. It has been determined that loss per 1 micron in diameter carbon particle is about 2.8.times.10.sup.-4 dB.
Carbon contamination can take place in the crucible in which the glass is made. During cooling of the melt in the crucible, certain regions of the crucible adhere to the side walls of the glass as the glass cools and contracts in volume. The carbon particles that chip off and find their way into preforms from which optical fibers are drawn are on the order of about 1 micron. On remelting the glass, the carbon particles are incorporated into the bulk and lead to a source of scattering in the preform and fibers made therefrom.
The known techniques for removing carbon film and carbon particles include (1) grinding and polishing the glass surface (2) chemically etching the glass surface, and (3) grinding, polishing and then etching the glass surface. These techniques are inappropriate because grinding leaves behind particles which cause light scattering and chemical etching causes water contamination leading to an increase in the OH peak absorbance.